It is known that thin film devices are generally quite sensitive to adverse environmental conditions. For example, humidity is one parameter in particular which is instrumental in causing detrimental functioning of thin film devices. The optical and electrical characteristics of thin film devices can be changed by chemical interaction and by heating, which parameters also effect the maintenance problems associated with thin film devices. It is, therefore, necessary to seal the active thin film device hermetically before it can be placed into operation in any environment.
Presently employed conventional methods for providing hermetic seals do provide some protection for the thin film electroluminescent devices. However, there are certain limitations and shortcomings relative to these present methods. In particular, they all tend to interact directly with the device during the sealing process.
One means of forming a seal is with the use of plastic or epoxy materials. Plastic and epoxy seals are themselves waterproof to a certain degree. However, increased humidity combined with increased temperature causes softening of many of the plastics and in most cases they tend to delaminate from the glass surfaces. This action is usually preceded by a deterioration of the tin oxide or indium tin oxide electrodes when the device is operated under humid conditions. There are numerous plastic products and also one and two component epoxies that are presently in use but all tend to fail after several hundred hours of use, particularly in a humid environment.
A second group of seals are glass-to-metal seals. In some ways these are an improvement over plastic seals but require higher processing temperatures even when soft glass substrates are used. If hard glass substrates are utilized for thin film deposition, the frit glass sealing temperatures are beyond the tolerable limit for dark field devices. Even the soft glass frits are only within a few degrees (sealing temperature of 400.degree. C.) of a point where the dark field characteristics change. The sealing temperature even for soft glass frits is at a temperature level where chemically absorbed water is desorbed, causing blistering and flaking of the thin film layers.
Another method of frit glass sealing is one in which a picture frame type layer of solder glass is screen printed and then fritted over the tin oxide layer of the thin film assembly. This sealing glass frame is then coated with nickel and can subsequently be soldered to a cover glass which is treated in the same fashion. Alternatively, an organo-metallic paint, such as silver-platinum, may be fired onto the sealing frame in place of the above-mentioned nickel. This process is carried out prior to any thin film deposition. The difficulty with this process is that the application is rather limited to relatively crude display devices because in the case of a dot matrix display, with pixels of only a few mils in size, the masking of the glass frame processed panels becomes virtually impossible.
It is believed, therefore, that an improved hermetically sealed enclosure for a thin film device which not only provides for proper sealing, but allows the device to operate for extended periods of time, under rugged conditions if necessary, would be deemed a significant advancement in the art.